1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory, and more particularly, to a nonvolatile semiconductor memory using tunnel effect to write data therein.
2. Description of the Related Art
Flash memory is a kind of nonvolatile semiconductor memory which should have a low cost, consume low power, and simply perform a write operation at high speed. If these requirements are met, flash memory will be used as data memory in portable equipment or as a replacement for a magnetic disk.
One prior art flash memory writes data by injecting hot electrons into memory cells. The write speed of this flash memory is very high. It can write one byte of data within several microseconds. This flash memory, however, requires a current of several milliamperes to write a byte. When simultaneously writing a large quantity of data of, for example, several bytes, the flash memory requires a large power source capacity and has high power consumption.
Another flash memory writes data by using the tunnel effect. The write speed of this flash memory is low. It takes several milliseconds to write one byte of data. This flash memory, however, requires a current of only several tens of nanoamps (nano amperes) to write a byte. Accordingly, this flash memory is capable of simultaneously writing a large quantity of data while requiring little power.
The tunnel-effect flash memory usually writes data page-by-page and each page consists of, for example, 512 bytes. After writing data into a page, the conventional flash memory verifies the data in the page by reading the data byte by byte, which is time-consuming.
The tunnel-effect flash memory is described in, for example, IEDM (International Electron Devices Meeting) of 1992, Report No. 24-3-1 p. 599 and Report No. 24-7-1, p. 991.
The tunnel-effect flash memory writes data page-by-page with each page containing 512 bytes. It is necessary for the flash memory to equalize the threshold voltage Vth of each memory cell into which data has been written. To achieve this, a write operation and a verify operation are alternately repeated, in order to gradually strengthen the written state until the threshold voltage Vth is obtained. To speedily complete a write operation in the flash memory, it is important to shorten the verify operation after the write operation. It is also important to shorten the verify operation after an erase operation which is carried out before the write operation. The problems of the flash memory of the prior art will be explained in detail with reference to FIGS. 1A and 1B.